Conventionally, a roller hemming device is widely known that is configured to perform bending (preliminary bending) of a standing flange of a work such as a door subassembly of a car at a predetermined angle by use of a roller, and then to perform bending (final bending) of the flange at a final angle by use of the roller.
In the roller hemming device mentioned above, during the preliminary bending, for the purpose of stability of an attitude of the roller, the roller is rolled along a guide surface formed on a bottom die on which the work is placed with the outer circumferential surface of the roller being in contact with the guide surface (e.g. see Patent Literature 1).
Consequently, a portion of the roller which comes in contact with the guide surface gradually abrades, which leads to difficulty in hemming the work in a constant condition.
In the present circumstances, to prevent defects resulted from abrasion of the roller, the roller having a predetermined quantity of abrasion is replaced with a new one. However, it is disadvantageous in that a running cost of the roller hemming device is high since the whole roller has to be replaced.